1. Field of the Invention
The invention relates to cleaning water for cleaning a wafer on which semiconductor devices are formed and a method of cleaning a wafer by using this cleaning water. In particular, the invention relates to cleaning water and a method of cleaning a wafer to be used in the final rinse of a wafer.
2. Description of the Related Art
In the fabrication of semiconductor devices, wafer cleaning steps are interposed between individual steps so as to remove particles and traces of impurities adhering to the wafer during the respective steps. With miniaturization of semiconductor devices, the size and concentration of particles in question have become smaller lately. Cleaning technologies are thus of ever-increasing importance. The size of particles in question recently is 0.1 xcexcm and above, so that control has come to be exercised on particles (dust) of such size. A large amount of particles existing on a wafer can cause detects such as a pattern defect and deteriorate the devices in yield.
FIG. 1 is a perspective view showing a method of cleaning a wafer according to conventional single-wafer spin processing. As shown in FIG. 1, a wafer 1 is rotated while pure water (not shown) is sprayed from a nozzle 2 to the central part of the surface of the wafer 1. The pure water falling on the central part of the surface of the wafer 1 moves toward the rim of the wafer 1 due to centrifugal force resulting from the rotation of the wafer 1. By this movement, the pure water cleans the surface of the wafer 1. Incidentally, the pure water may be referred to as DIW (de-ionized water) or ultra-pure water.
The foregoing conventional technology, however, has the following problems. When pure water is used to clean the wafer 1, thin gate oxide films in the central part of the cleaned wafer 1 may be broken. Given that the wafer 1 is provided with wiring portions exposing large areas of Cu, thin Cu lines drawn from the wiring portions become easier to be eluted as the exposed areas increase. Moreover, particles tend to concentrate on the central part. Furthermore, the periphery of the wafer becomes prone to Cu oxidation or elution. The causes of these problems have been heretofore unknown.
It is an object of the present invention to provide cleaning water for cleaning a wafer and a method of cleaning a wafer that can avoid the breakdown of gate oxide films and the particle concentration in the central part of the wafer as well as the oxidation and elution of metal films formed on the wafer.
Cleaning water for cleaning a wafer according to the present invention is used for cleaning a wafer in a step of rinsing the wafer in single-wafer spin processing. This cleaning water has a resistivity of not greater than 1 Mxcexa9xc2x7cm and a pH of 7.5 to 9, contains ammonium hydroxide, tetramethylammonium hydroxide, or choline, and is reducing water.
According to the present invention, the cleaning water given a resistivity of not greater than 1 Mxcexa9xc2x7cm can preclude static electricity from occurring due to friction between the cleaning water and the wafer in the cleaning step of single-wafer spin processing. This makes it possible to avoid the breakdown of gate oxide films formed on the central part of the wafer and the particle concentration on the central part of the wafer that are ascribable to the static electricity. In addition, the cleaning water is made weakly alkaline, or 7.5 to 9 in pH, and is reducing water. This can suppress Cu oxidation and elution. Moreover, containing ammonium hydroxide (NH4OH), tetramethylammonium hydroxide, or choline can make the cleaning water alkaline, and suppress residuals to appear on the wafer when the wafer is cleaned and then dried. Consequently, this cleaning water can be used in the final rinse as a substitute for pure water. In particular, the cleaning water preferably contains ammonium hydroxide which is extremely volatile and the least liable to remain.
Moreover, the cleaning water preferably may have an oxidation-reduction potential of xe2x88x920.7 to xe2x88x920.2 V and contain hydrogen gas of 0.2 to 5 ppm in concentration. This allows a further reduction of the Cu elution. It is yet preferable that the cleaning water falls within a range of xe2x88x920.6 and xe2x88x920.45 V in oxidation-reduction potential and the range of 1 and 2.5 ppm in the concentration of hydrogen gas.
A method of cleaning a wafer according to the present invention is a method of cleaning a wafer in a rinse step of single-wafer spin processing. In this method of cleaning a wafer, the wafer is rotated about an axis perpendicular to a surface of this wafer while cleaning water is sprayed onto the surface of the wafer to clean the surface. This cleaning water has a resistivity of not greater than 1 Mxcexa9xc2x7cm and a pH of 7.5 to 9, contains ammonium hydroxide, tetramethylamonium hydroxide, or choline, and is reducing water. Incidentally, the surface of the wafer shall refer to both the side on which semiconductor devices etc. are formed and the side opposite thereto.
According to the present invention, accumulation of charges on the wafer can be suppressed in the step of spin-cleaning the wafer. It is therefore possible to preclude the breakdown of gate oxide films and the concentration of particles in the central part of the wafer. It is also possible to avoid the elution of metal films that constitute the wiring, electrodes, and the like formed on the wafer.